Do the same as commit f8f0d0b6 ("mm: ptdump: reduce level numbers
by 1 in note_page()") and add missing p4d level.

This will align powerpc to the users of generic ptdump.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d76495c574132b197b445a1f133755cca4b912a4.1625762906.git.christophe.leroy@csgroup.eu